The present invention relates to a sealed alkaline storage battery including a positive electrode active material and a negative electrode active material packed in total in a battery can at 75% by volume or more of the content volume of the battery can. More particularly, it relates to improvement of a positive electrode active material for the purpose of providing a highly reliable sealed alkaline storage battery from which an electrolyte hardly leaks for a long period of charge-discharge cycles.
Manganese dioxide has been proposed as a positive electrode active material for a sealed alkaline storage battery using zinc as a negative electrode active material (Japanese Patent Publication No. 45-3570). Also, a mixture of nickel oxide and manganese dioxide has been proposed as a positive electrode active material for an alkaline primary battery using zinc as a negative electrode active material (Japanese Laid-Open Patent Publication No. 49-114741).
However, manganese dioxide is poor in reversibility in a charge-discharge reaction and is difficult to return to manganese dioxide by charge after discharge. Therefore, the utilization of the active material is rapidly lowered through repeated charge-discharge cycles, resulting in rapidly decreasing the discharge capacity. Furthermore, the oxygen evolution potential of manganese dioxide is so low that the pressure within the battery is increased due to an oxygen gas generated through decomposition of water on the positive electrode during charge. As a result, the adhesion in a connecting portion of a battery housing member is degraded, so that the electrolyte can easily leak.
On the other hand, when the mixture of nickel oxide and manganese dioxide is used in a storage battery (secondary battery) that is repeatedly charged and discharged, the oxygen evolution potential of the mixture is so low that the pressure within the battery can be easily increased during charge and the electrolyte can easily leak similarly to the battery using manganese dioxide. Furthermore, manganese dioxide included in the mixture is poor in reversibility in a charge-discharge reaction, and hence, the utilization of the active material is rapidly lowered through repeated charge-discharge cycles, resulting in rapidly decreasing the discharge capacity.
In this manner, both of the positive electrode active materials are too disadvantageous to be used as a positive electrode active material for a sealed alkaline storage battery. The increase of the pressure within a battery during charge and the resultant leakage of the electrolyte are particularly significant in a sealed alkaline storage battery including an active material in a large amount.
Accordingly, an object of the invention is providing a highly reliable sealed alkaline storage battery including an active material in a large amount but hardly suffering electrolyte leakage for a long period of charge-discharge cycles.
Another object of the invention is providing a sealed alkaline storage battery that can keep high utilization of an active material not only in initial stages of charge-discharge cycles but also for a long period of time.
The sealed alkaline storage battery (hereinafter referred to as the xe2x80x9cfirst batteryxe2x80x9d) according to claim 1 (first invention) comprises a negative electrode of a zinc electrode, a cadmium electrode or a hydrogenated hydrogen-absorbing alloy electrode; and a positive electrode active material and a negative electrode active material packed in total in a battery can at 75% by volume or more of a content volume of the battery can, and the positive electrode active material includes 60 through 100 wt % of nickel oxyhydroxide including Mn as a solid-solution element and having a xcex3 ratio defined as follows of 65 through 100%, and 40 through 0 wt % of xcex1-Ni(OH)2:
xcex3 ratio (%)={S1/(S1+S2)}xc3x97100
in which S1 indicates a peak area in a lattice plane (003) in an X-ray diffraction pattern of the nickel oxyhydroxide including Mn as a solid-solution element; and S2 indicates a peak area in a lattice plane (001) in the X-ray diffraction pattern of the nickel oxyhydroxide including Mn as a solid-solution element.
The peak area S1 in the lattice plane (003) in the above-described formula corresponds to the amount of xcex3-nickel oxyhydroxide included in the nickel oxyhydroxide, and the peak area S2 in the lattice plane (001) in the formula corresponds to the amount of xcex2-nickel oxyhydroxide included in the nickel oxyhydroxide. Accordingly, the xcex3 ratio corresponds to the proportion (%) of xcex3-nickel oxyhydroxide in the nickel oxyhydroxide.
The first battery uses the positive electrode active material including 65 through 100 wt % of nickel oxyhydroxide including Mn as a solid-solution element and having a xcex3 ratio of 65 through 100%, and 40 through 0 wt % of xcex1-Ni(OH)2. When the proportion of the nickel oxyhydroxide including Mn as a solid-solution element is smaller than 60 wt %, namely, when the proportion of xcex1-Ni(OH)2 exceeds 40 wt %, the oxygen overvoltage of the positive electrode becomes too low to obtain a sealed alkaline storage battery hardly suffering electrolyte leakage for a long period of charge-discharge cycles. Also, when the xcex3 ratio is smaller than 65% and a large amount of xcex2-nickel oxyhydroxide is included, the oxygen overvoltage of the positive electrode is so low that an oxygen gas can be easily generated. The xcex3 ratio is preferably 90 through 100%.
The nickel oxyhydroxide including Mn as a solid-solution element can be obtained by oxidizing nickel hydroxide including Mn as a solid-solution element with an oxidizing agent. Examples of the oxidizing agent are sodium hypochlorite, potassium permanganate and potassium persulfate. A desired xcex3 ratio can be attained by increasing/decreasing the amount of the oxidizing agent to be added. When a larger amount of the oxidizing agent is added, a higher xcex3 ratio is attained.
The nickel oxyhydroxide including Mn as a solid-solution element preferably has a Mn ratio defined as follows of 5 through 50%:
Mn ratio (%)={M/(M+N)}xc3x97100
wherein M indicates the number of Mn atoms included in the nickel oxyhydroxide including Mn as a solid-solution element; and N indicates the number of Ni atoms included in the nickel oxyhydroxide including Mn as a solid-solution element.
When the Mn ratio is lower than 5%, the oxygen overvoltage (oxygen evolution potentialxe2x88x92charge potential) cannot be sufficiently increased by adding Mn as a solid-solution element to nickel oxyhydroxide, and hence, an oxygen gas can be easily generated on the positive electrode. On the other hand, when the Mn ratio is higher than 50%, Mn cannot be completely dissolved as a solid-solution element in nickel oxyhydroxide, and hence, a generated free Mn oxide obstructs the discharge.
The Mn ratio is equal to the proportion (%) of the number of Mn atoms included in the nickel hydroxide including Mn as a solid-solution element to the total number of Mn atoms and Ni atoms. Accordingly, nickel oxyhydroxide having a desired Mn ratio can be obtained by adjusting the amounts of a Mn material (such as manganese sulfate) and a Ni material (such as nickel sulfate) to be mixed for preparing nickel hydroxide including Mn as a solid-solution element.
The first invention is applied to a sealed alkaline storage battery including the active materials packed in total in the battery can at 75% by volume or more of the content volume of the battery can for the following reason: Particularly in a sealed alkaline storage battery including a large amount of active materials packed in a battery can, the pressure within the battery is easily increased, and the electrolyte can easily leak during repeated charge-discharge cycles. The increase of the pressure within the battery can be remarkably suppressed by using the positive electrode active material having a high oxygen overvoltage according to the first invention.
The first invention is applicable to, for example, a sealed alkaline storage battery that uses zinc, cadmium or a hydrogenated hydrogen-absorbing alloy as a negative electrode active material and does not need charge before use.
Since the first battery uses, as the positive electrode active material, the nickel oxyhydroxide including Mn as a solid-solution element and having a large xcex3 ratio, the pressure within the battery is less increased during charge, and hence, the electrolyte hardly leaks for a long period of charge-discharge cycles.
The sealed alkaline storage battery (second battery) according to claim 6 (second invention) comprises a positive electrode active material of nickel oxyhydroxide; and a negative electrode of a zinc electrode, a cadmium electrode or a hydrogenated hydrogen-absorbing alloy electrode, and the positive electrode active material and a negative electrode active material are packed in total in a battery can at 75% by volume or more of a content volume of the battery can, and the nickel oxyhydroxide includes, as an additive, at least one rare earth element and/or at least one rare earth compound in a ratio of the rare earth element to the nickel oxyhydroxide of 0.05 through 5 wt %.
Furthermore, the sealed alkaline storage battery (third battery) according to claim 10 (third invention) comprises a positive electrode active material of nickel oxyhydroxide; and a negative electrode of a zinc electrode, a cadmium electrode or a hydrogenated hydrogen-absorbing alloy electrode, and the positive electrode active material and a negative electrode active material are packed in total in a battery can at 75% by volume or more of a content volume of the battery can, and the nickel oxyhydroxide includes, as a coat layer formed on a particle surface, at least one rare earth element and/or at least one rare earth compound in a ratio of the rare earth element to the nickel oxyhydroxide of 0.05 through 5 wt %.
The second and third inventions are applied to a sealed alkaline storage battery including the active materials packed in total in the battery can at 75% by volume or more of the content volume of the battery can for the following reason: Particularly in a sealed alkaline storage battery including a large amount of active materials packed in a battery can, the pressure within the battery is easily increased, and the electrolyte can easily leak during repeated charge-discharge cycles. The increase of the pressure within the battery can be remarkably suppressed by using the positive electrode active material having a high oxygen overvoltage according to the second or third invention.
In the second battery, the nickel oxyhydroxide includes, as an additive, at least one rare earth element and/or at least one rare earth compound in a ratio of the rare earth element to the nickel oxyhydroxide of 0.05 through 5 wt %, and in the third battery, the nickel oxyhydroxide includes, as a coat layer formed on a particle surface, at least one rare earth element and/or at least one rare earth compound in a ratio of the rare earth element to the nickel oxyhydroxide of 0.05 through 5 wt %. When the amount of a rare earth element and/or a rare earth compound to be included as the additive or as the coat layer is smaller than 0.05 wt % in a ratio of the rare earth element to the nickel oxyhydroxide, the oxygen overvoltage of the positive electrode cannot be sufficiently increased, and hence, the evolution of an oxygen gas during charge cannot be sufficiently suppressed. On the other hand, when the amount to be included as the additive or as the coat layer exceeds 5 wt %, the amount of the nickel oxyhydroxide to be packed as the active material is decreased so that the discharge capacity can be lowered.
The rare earth element is a general term for 17 elements: scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). Examples of the rare earth compound are an oxide, a hydroxide, a fluoride and a carbonate. For the purpose of increasing the oxygen overvoltage of the positive electrode, yttrium, erbium and ytterbium are preferred as the rare earth element, and an yttrium compound, an erbium compound and an ytterbium compound are preferred as the rare earth compound.
The nickel oxyhydroxide serving as the positive electrode active material preferably has a valence of nickel of 3.0 through 3.8 when fully charged. When the nickel oxyhydroxide has a valence of nickel smaller than 3.0, a sufficient discharge capacity is difficult to attain. No nickel oxyhydroxide has a valence of nickel larger than 3.8. Even when the battery is continuously charged after being fully charged, merely an oxygen gas is generated through decomposition of water, and the valence of nickel never exceeds 3.8.
The nickel oxyhydroxide is obtained, for example, by oxidizing nickel hydroxide with an oxidizing agent such as sodium hypochlorite (NaClO).
The nickel oxyhydroxide can include, as a solid-solution element, at least one element selected from the group consisting of manganese (Mn), zinc (Zn), cobalt (Co), bismuth (Bi) and rare earth elements. When the nickel oxyhydroxide including any of these elements as a solid-solution element is used, the oxygen overvoltage of the positive electrode can be further increased. The nickel oxyhydroxide preferably has a ratio of a solid-solution element defined as follows of 5 through 50%:
Ratio of solid-solution element (%)={X/(X+N)}xc3x97100
wherein X indicates the number of atoms of the solid-solution element included in the nickel oxyhydroxide, and N indicates the number of nickel atoms included in the nickel oxyhydroxide.
When the ratio of a solid-solution element is too small, the oxygen overvoltage of the positive electrode cannot be effectively increased. When the ratio of a solid-solution element is too large, the amount of the nickel oxyhydroxide to be packed in a given volume is decreased, resulting in lowering the discharge capacity.
The coat layer formed in the third battery can be obtained, for example, by adding a nickel hydroxide powder to an aqueous solution of a salt of a rare earth element, adjusting the pH of the resultant solution by adding a sodium hydroxide aqueous solution with stirring, and stirring the solution for 30 through 60 minutes, so as to chemically precipitating the rare earth element as a hydride on particle surfaces of nickel hydroxide. The amount used for coating can be adjusted by changing the concentration of the aqueous solution of the salt of the rare earth element or the proportion thereof to the nickel hydroxide powder. The coat layer can also be formed by a mechanical charge method for dry blending a nickel hydroxide powder and a rare earth element and/or a rare earth compound in a non-oxidizing atmosphere. Examples of the non-oxidizing atmosphere are atmospheres of an inert gas, hydrogen, nitrogen and vacuum. The oxidation of nickel hydroxide can be conducted before forming the coat layer or after forming the coat layer.
The positive electrode of the second or third battery includes a predetermined amount of a rare earth element and/or a rare earth compound, and hence has a high oxygen overvoltage. Accordingly, the pressure within the battery is less increased during charge, and the electrolyte hardly leaks for a long period of charge-discharge cycles.
Moreover, the sealed alkaline storage battery (fourth battery) according to claim 14 (fourth invention) comprises a positive electrode active material of nickel oxyhydroxide; and a negative electrode of a zinc electrode, a cadmium electrode or a hydrogenated hydrogen-absorbing alloy electrode, and the positive electrode active material and a negative electrode active material are packed in total in a battery can at 75% by volume or more of a content volume of the battery can, and the nickel oxyhydroxide has a half-width of a peak in a lattice plane (003) in an X-ray diffraction pattern of 0.8xc2x0 or more.
The fourth invention is applied to a sealed alkaline storage battery including the active materials packed in total in the battery can at 75% by volume or more of the content volume of the battery can because the increase of the pressure within the battery derived from decrease in the utilization of the active material is particularly serious in a sealed alkaline storage battery including a large amount of active materials packed in the battery can.
As the crystallinity of nickel oxyhydroxide is poorer, the half-width of the peak in the lattice plane (003) in the X-ray diffraction pattern is larger and the peak is broader. In this invention, in order to improve the utilization of the active material, the nickel oxyhydroxide with poor crystallinity having a half-width of the peak in the lattice plane (003) in the X-ray diffraction pattern of 0.8xc2x0 or more is used as the positive electrode active material. Herein, a half-width of a peak means a peak width at a half height of the peak from a base line.
Nickel oxyhydroxide (NiOOH) is changed into nickel hydroxide (Ni(OH)2) by discharge, and the nickel hydroxide generated through the discharge is changed into nickel oxyhydroxide by charge. Protons (H+) are released from nickel oxyhydroxide into the electrolyte during discharge, and the released protons are absorbed by nickel hydroxide during charge. Accordingly, in order to sufficiently use nickel oxyhydroxide in the charge-discharge reaction, protons should easily move in the nickel oxyhydroxide. The nickel oxyhydroxide where protons can easily move is nickel oxyhydroxide with poor crystallinity. This is because the nickel oxyhydroxide with poor crystallinity having a half-width of the peak in the lattice plane (003) in the X-ray diffraction pattern of 0.8xc2x0 or more is used in this invention.
Nickel oxyhydroxide can be obtained, for example, by oxidizing nickel hydroxide with an oxidizing agent such as sodium hypochlorite (NaClO). Nickel hydroxide can be obtained, for example, as a precipitate by mixing an alkaline aqueous solution (such as a sodium hydroxide aqueous solution) with an aqueous solution of a salt of nickel (such as a nickel sulfate aqueous solution). The crystallinity of nickel hydroxide can be adjusted by adjusting the pH of the mixed solution used for precipitating nickel hydroxide. As the pH of the mixed solution is lower, the crystallinity of the resultant nickel hydroxide is poorer. Accordingly, the crystallinity of nickel oxyhydroxide obtained by oxidizing this nickel hydroxide is also poorer.
The nickel oxyhydroxide can include, as a solid-solution element, at least one element selected from the group consisting of bismuth (Bi), cadmium (Cd), cobalt (Co), magnesium (Mg), manganese (Mn), yttrium (Y) and zinc (Zn). When nickel oxyhydroxide includes any of these elements as a solid-solution element, the nickel oxyhydroxide can be suppressed from swelling. The nickel oxyhydroxide preferably has a ratio of a solid-solution element of 5 through 50%.
When the ratio of a solid-solution element is too small, the nickel oxyhydroxide cannot be effectively suppressed from swelling. When the ratio of a solid-solution element is too large, the amount of nickel oxyhydroxide to be packed is decreased, resulting in lowering the discharge capacity.
Since the fourth battery uses, as the positive electrode active material, the nickel oxyhydroxide with poor crystallinity in which protons can easily move during charge-discharge, the utilization of the active material can be kept high for a long period of charge-discharge cycles. Also, since the utilization of the active material can be kept high for a long period of charge-discharge cycles, the pressure within the battery is less increased during charge, and hence, the electrolyte hardly leaks for a long period of charge-discharge cycles.